Carbon-carbon and/or metal-carbon fiber composite heat spreaders

ABSTRACT

A heat spreader, comprised of a plurality of carbon fibers oriented in a plurality of directions, with a carbon or metal matrix material dispersed about the fibers, is described. The carbon fibers facilitate the spreading of heat away from the smaller semiconductor device and up to a larger heat removal device, such as a heat sink.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This invention relates to semiconductor manufacturing technologygenerally, and more specifically, to heat spreader technology for heatdissipation in a semiconductor package.

[0003] 2. Description of the Related Art

[0004] There is a trend toward increasing the number of functions builtinto a given integrated circuit (also referred to herein as a device).This results in an increasing density of circuits in the device. Alongwith the increased circuit density, there is always a desire to increasethe data processing rate; therefore, the clock speed of the device isincreased as well. As the density of circuits and the clock speedincrease, the amount of heat generated by the device increases.Unfortunately, device reliability and performance will decrease as theamount of heat that the device is exposed to increases. Therefore, it iscritical that there be an efficient heat-removal system associated withintegrated circuits.

[0005]FIG. 1 illustrates a typical integrated circuit and associatedpackaging. There are a number of methods for removing heat fromintegrated circuits 103, including active methods, such as fans orrecirculated coolants (not shown), or passive methods, such as heatsinks 107 and heat spreaders 105. Because of the decreasing device 103size, there is usually a need to evenly distribute heat generated by thesmall device 103 to the larger heat sink 107 to eliminate “hot spots” inthe device. This is the function of heat spreaders 105. Heat spreadersare coupled to the integrated circuit 103 through the use of a thermallyconductive material 104. These thermal interface materials 104, such asgel or grease containing metal particles to improve heat conduction, areapplied in between the device 103 and the heat spreading structure 105to improve the heat transfer from the integrated circuit 103 to the heatspreader 105. Typically, the heat spreading structure 105 will beconstructed either of a ceramic material or a metal, such as aluminum orcopper. Aluminum is preferred from a cost standpoint, as it is easy andcheap to manufacture; however, as the heat load that needs to betransferred increases, copper becomes the metal of choice because of itssuperior heat transfer characteristics (the thermal conductivity for Alis ˜250 W/m·K vs ˜395 W/M·K for Cu.) There will typically be acontiguous wall 106 around the periphery of the heat spreader, whichserves as a point of attachment and support between the substrate 101and the spreader 105. There is often a heat sink 107 attached to theheat spreader 105, to allow for the greater cooling capacity associatedwith the high-surface area of the heat sink 107.

[0006] With increased heat dissipation requirements, it has becomenecessary to improve heat spreader 105 and/or heat sink 107 performance.While improving heat sink performance through active cooling methodssuch as fans or recirculated liquids works well, there are a number ofdisadvantages associated with this solution, including bulkiness, costand noise.

[0007] A second method for increasing heat dissipation capacity forintegrated circuit packaging is through improvement of heat spreaderperformance. Current heat spreader materials allow for heat conductionin the range of 80-400 W/m-° K. FIG. 2 illustrates one method ofincreasing the rate of heat conduction in heat spreaders 201 a, 201 b.FIG. 2a shows a top view of a heat spreader 201 a while FIG. 2billustrates a cross-section of the same heat spreader 201 b. Compositesusing layers of highly conductive carbon fibers 202 a, 202 b impregnatedwith carbon resin or metals 203 a, 203 b are known to be very effectiveconductors of heat. These materials also offer the added advantage oflighter weight as compared to the present materials (e.g. a density of5.9 g/cc for Cu matrix composite versus 8.9 g/cc for copper), decreasingpackaging weight, shipping cost and offering ergonomic advantages formanufacturing personnel. However, these materials have suffered from thedisadvantage of being anisotropically oriented in their heat flow, thusthey are typically highly conductive (>500 W/m-° K.) in only onedirection. The direction of heat conduction follows the longitudinalorientation of carbon fibers, therefore the unidirectional heat flow isa result of the majority of the fibers in the composite being orientedin one direction. The aforementioned advantages are often outweighed bythe disadvantage of poor heat conduction in both the second horizontaland the vertical directions.

[0008] Therefore, what is needed is an apparatus for increasing the rateof heat transfer in all three directions, allowing the rapid dissipationof heat through the heat spreader and to the heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is illustrated by way of example, and notlimitation, in the Figures of the accompanying drawings in which:

[0010]FIG. 1 shows a prior art integrated circuit package design.

[0011]FIG. 2a shows a prior art heat spreader design.

[0012]FIG. 2b shows a cross-section of a prior art heat spreader design.

[0013]FIG. 3 shows a cross section of an integrated circuit packagecontaining an embodiment of a heat spreader using carbon fibers tospread heat in multiple dimensions.

[0014]FIG. 4a shows a top view of an embodiment of a heat spreader usingcarbon fibers to spread heat in multiple dimensions

[0015]FIG. 4b shows a cross-section of an embodiment of a heat spreaderusing carbon fibers to spread heat in multiple dimensions

[0016]FIG. 5 shows a different embodiment of a heat spreader usingcarbon fibers to spread heat in multiple dimensions.

[0017]FIG. 6 shows an embodiment of a heat spreader using carbon fibersto spread heat in multiple dimensions where there are thermal interfacelayers on the top and bottom of the heat spreader to modify heatdissipation efficiency.

[0018]FIG. 7 shows an embodiment of a heat spreader using carbon fibersto spread heat in multiple dimensions that contains chopped fibers inbetween the horizontal fiber layers.

[0019]FIG. 8a shows top and side views of an embodiment of a heatspreader using carbon fibers to spread heat in multiple dimensions thathas attached standoffs.

[0020]FIG. 8b shows top and side views of differing embodiments ofstandoffs that can be used on the heat spreader of FIG. 8a.

[0021]FIG. 9 shows an integrated circuit package, containing anembodiment of a heat spreader using carbon fibers to spread heat inmultiple dimensions, which contains a plurality of integrated circuits.

DETAILED DESCRIPTION

[0022] An apparatus for increasing the rate of heat flow through a heatspreader is described. In the following description, numerous specificdetails are set forth such as material types, dimensions, etc., in orderto provide a thorough understanding of the present invention. However,it will be obvious to one of skill in the art that the invention may bepracticed without these specific details. In other instances, well-knownelements and processing techniques have not been shown in particulardetail in order to avoid unnecessarily obscuring the present invention.

[0023] A heat spreader, comprised of a plurality of carbon fibersoriented in a plurality of directions, with a carbon or metal matrixmaterial dispersed about the fibers, is described. The carbon fibersfacilitate the spreading of heat away from the smaller semiconductordevice and up to a larger heat removal device, such as a heat sink.

[0024] This discussion will mainly be limited to those needs associatedwith removing heat from the backside of a flip chip that is housedwithin a SMT or INT package. It will be recognized, however, that suchfocus is for descriptive purposes only and that the apparatus andmethods of the present invention are applicable to other types ofelectronic devices and other types of packaging.

[0025]FIG. 3 illustrates a cross-section view of a semiconductor packagein one embodiment of the present invention. The package includes asubstrate 301 having a semiconductor device 303 mounted on a top surfaceof the substrate 301. In one embodiment the substrate 301 is a printedcircuit board. In another embodiment, the substrate 301 may be adifferent material, such as silicon or ceramic.

[0026] In one embodiment, the semiconductor device 303 is mechanicallyand electrically coupled to the top surface of the substrate via aplurality of solder bump connections 302. In an embodiment the gap maybe filled with an epoxy underfill material (not shown). The substrate301 contains at least one wiring layer (not shown) that electricallyconnects the device to pins or balls located along the bottom surface ofthe substrate 301.

[0027] In accordance with the present invention, a composite heatspreader 305 is thermally coupled to the bottom of the flip chipstructure 302, 303 through a compliant heat-transfer medium 304. In oneembodiment, the heat transfer medium is thermal grease. In anotherembodiment, gel or other proprietary formulations may be used.

[0028] The heat spreader is further attached to the substrate using asealant material 307. The sealant material 307 surrounds the device 303and fills the gap between the substrate 301 and the heat spreader 305,forming a completely enclosed cavity containing the device 303. The useof the sealant material 307 allows for a more flexible bond between thesubstrate 301 and the heat spreader 305. In one embodiment the sealantmaterial may be silicone or other proprietary sealant material. Theflexible bond may help to compensate for differing coefficients ofthermal expansion (CTE) between the heat spreader and the substrate,resulting in a more consistent heat conduction pathway. A secondadvantage of the current embodiment is that the sealant is much lighterin weight compared to the metal used in the prior art contiguous wall(See FIG. 1, 106) design, resulting in a lighter package.

[0029] Next, a heat sink 306 is attached to the heat spreader 305 usinga thermal interface material 308. In one embodiment, the thermalinterface material 308 is thermal grease. The heat sink 306 should allowfor the more rapid dissipation of heat due to increased surface area forcooling, as discussed in the Background section above.

[0030]FIGS. 4a (top view) and 4 b (cross-sectional view) furtherillustrate the heat spreader of FIG. 3. The heat spreader 305 a, 305 bis formed from a composite material comprised either of carbon fibers402 a, 404 a, 405 a, 402 b, 404 b, 405 b impregnated with a resinmaterial 403 a, 403 b (referred to as a carbon/carbon composite) orcarbon fibers 402 a, 404 a, 405 a, 402 b, 404 b, 405 b impregnated witha metal or metal alloy 403 a, 403 b (referred to as a metal/carboncomposite). In one embodiment of the present invention, a carbon/coppercomposite material is used. However, in another embodiment, thecomposite may use a different thermally conductive matrix metal, such asaluminum or magnesium, a metal alloy, a ceramic, such as siliconcarbide, or an organic material, such as resin.

[0031] One factor in the choice of what composite material to use may bewhat material had been used previously in the packaging process.Matching the metal/composite material with the previous heat spreadermaterial may allow the use of the same adhesive or thermal grease systemas previously used, thus simplifying the conversion process from oneheat spreader material to another. A second factor to consider inchoosing the type of composite material to use is the CTE of thesubstrate material. It may be possible to better match the CTE of theheat spreader with that of the substrate, allowing for the production ofa more reliable package.

[0032] In the present embodiment, the heat spreader contains horizontallayers of fiber bundles 402 a, 404 a and 402 a, 404 b, consisting of twoperpendicular sets of fiber bundles woven into a sheet, oriented in thex-y plane of the apparatus. These woven fiber bundles facilitate heatconduction in the x-y plane. In addition, there is a second set of fiberbundles 405 a, 405 b, oriented substantially perpendicular to the firstset. The substantially perpendicular fiber bundles facilitate theconduction of heat in the z-direction.

[0033] In the present embodiment, the fiber bundles are comprised ofapproximately 1000 individual carbon fibers twisted into a fiber bundle.These bundles of fibers are then woven into a sheet. The weave in thepresent invention should be balanced, to produce a flatter heatspreader. The weave may be balanced by attempting to ensure that thefibers throughout the x-y plane of the woven mat have substantially thesame number of downward stitches as up. In one embodiment, theindividual carbon fibers have a diameter of approximately 10 microns,with a density of about 2.2 g/cc. In this embodiment, the fibers mayhave a thermal conductivity as high as 1000 W/m·K. One example ofcommercially available carbon fiber is Amoco K1100 2K™. While carbonfibers are discussed in this embodiment, other types of highlyconductive (>500 W/m·K) fibers or wires, based on materials such aspolymers, metals or ceramics, may work in the present invention. In adifferent embodiment, the fibers may have very different physical anddimensional properties, and the above thermal and physical propertiesshould not be construed as limiting the properties of the fibrousmaterials used.

[0034] The woven fiber sheets are then impregnated with a metal, metalalloy, carbon or ceramic matrix material, as discussed above. The matrixmaterial may be dispersed about the woven sheets using a number ofdifferent methods. In this embodiment, a compression molding method isused. In another embodiment, injection molding or any of a multitude ofmolding processes practiced in the art may be used. The molded materialmay be allowed to cure, and then, if necessary, could be cut to thecorrect dimensions. In an embodiment, a laser may be used to cut thecomposite material. In another embodiment, a mechanical means, such as asaw or mill, may be used.

[0035] The heat spreader in the current invention provides for betterconduction of heat in the z-direction, with a possible thermalconductivity in the range of 500-1000 W/m·K. In the present embodiment,a finned heat sink can be attached to the top surface of the heatspreader. Through the use fibers oriented in the z-direction, heat canbe conducted up through the heat spreader to the heat sink, and the heatcan be dissipated to the surrounding environment, with cooling for thesink provided by the surrounding air or an active cooling method, asdiscussed in the Background section.

[0036] In addition, the fibers oriented in the x-y plane 402 a, 402 b,403 a, and 403 b allow the heat to dissipate radially, thus preventingthe formation of localized hot spots. Localized heating decreases thearea available for heat transfer, which decreases the overall heat fluxfrom the device. The fibers in the x-y plane 402 a, 402 b, 403 a, and403 b allow the conducted heat to rapidly dissipate from the relativelysmall contact area in the point of attachment of the device to the heatspreader, over virtually the entire (larger) area of the heat spreader.This means that heat can be removed much more efficiently.

[0037]FIG. 5 illustrates that the orientation of the fibers in thez-direction may differ from the essentially perpendicular orientationabove. In this embodiment, the fibers in the z-axis 502 may be orientedat approximately +/−30 degrees from fibers in the x-y plane 503. It isto be understood, however, that the relative orientation in thez-direction may comprise any angle between 0 and +/−90 degrees from thex-y plane.

[0038] In a third embodiment of the invention, it may be desirable toinclude a higher density thermal interface layer on the top and bottomsurface of the heat spreader, as shown in FIG. 6. In this embodiment,the thermal interface layer 601 could comprise the same carbon fibermaterial used in the above invention 603, only with an increased fiberdensity from that used in the main body of the heat spreader 603. In anembodiment, the fiber density in the thermal interface layer 601 may befour times that of the main body 603. While this embodiment may use thesame fiber as the above embodiments, it is understood that otherthermally conductive materials may also be used as the thermal interfacematerial, including non-composite materials. In addition, thehigher-density carbon fiber layer may be comprised of chopped fibers, asdiscussed in FIG. 7 below. In an embodiment, the increased fiber density601 may allow for even greater heat conduction capability, and may beused to more rapidly dissipate heat in the x-y direction from hot spotsin the device 303/heat spreader 305 interface region, in addition toallowing more rapid conduction from the heat spreader 303 to the heatsink (not shown) through the top layer 602.

[0039] In another embodiment of the present invention, the fiberspredominantly oriented in the z-direction are replaced with choppedfibers. In an embodiment, the chopped fibers are comprised of carbonfiber that has been broken or cut, using a mechanical means, intosegments less than 0.5 mm long. However, the length of the chopped fibermay vary considerably from this length, and the aforementioneddimensions should not be construed as limiting the allowable choppedfiber length. In addition, another embodiment may use some other type ofhighly conductive fibrous material, rather than carbon fiber, asdiscussed above in FIG. 4.

[0040] In this embodiment, shown in FIG. 7, the chopped fibers 701 areplaced in between the woven sheets 702 that are oriented in the x-yplane, forming a layered structure. The chopped fiber 701 and the wovensheets 702 are then impregnated with a carbon or metal-based matrixmaterial 703, to form a composite. The chopped fibers 701 may aid inincreasing thermal conductivity in the z-axis, much like the orientedfibers discussed in previous embodiments. The use of chopped fibers 701may allow the manufacture of a heat spreader with many of the advantagesdiscussed above at a reduced cost. In a further embodiment, the wovensheets 702 may be eliminated all together, and chopped fibers 701 orother type of carbon material, such as carbon flakes, dispersedthroughout the heat spreader may be used to facilitate conductionthrough the matrix.

[0041] Recall from the discussion of FIG. 3 that the heat spreader 305is attached to the substrate 301 using a sealant material 307. Thisallowed for a more flexible and lighter weight point of attachmentbetween the two structures. Sometimes, however, a more rigid package maybe desired. FIGS. 8a and 8 b illustrate how a more rigid structure maybe achieved with the present invention. In this embodiment, a pluralityof legs 802 a are added to the heat spreader 305 a to serve asadditional support and points of attachment when bonding the heatspreader 305 a to the substrate (301 in FIG. 3.) In this illustration,four cylindrical legs are shown. It is to be understood that the numberof legs, and their shape, may vary from application to application.Referring to FIG. 8b, other shapes may include, but are not limited to,rectangular legs 801 b, contiguous walls 802 b, non-contiguous walls 803b, rectangular legs with holes 804 b or rectangular legs with feet 805b. The relationship of the substrate to these structures is shown by 806b. In an embodiment, the above structures are approximately 0.63 mmtall. It should be understood, however, that the height of thesestructures will vary with different applications, and that the abovedimension should not be construed as limiting the size of thestructures.

[0042] Referring again to FIG. 8A, the legs 802 a may be constructedusing a variety of materials and methods. In one embodiment, they areconstructed of a polymeric material. One example of such as a materialis a high modulus epoxy.

[0043] One method of manufacturing legs using high modulus epoxies wouldbe injection molding. However, there are a multitude of other methodsthat may be used for forming the legs, depending on the material used.These include, but are not limited to: Machining, liquid resin moldingand thermoforming.

[0044] The legs 802 a may be attached to the heat spreader 305 a througha bonding process. Examples of bonding types may include attachment withan adhesive, such as an epoxy, or soldering. Depending on the type ofmaterial used for the legs and the type of bonding process, it may benecessary to roughen the surface of the heat spreader at the point ofattachment, to increase the strength of the foot/heat spreader bond.Although there are a multitude of methods that can be used, examples oftechniques used for roughening may include mechanical means, or throughlaser marking.

[0045] While the previous embodiments have focused on flip chip packagescontaining a single device, the present invention could also be used forpackaging substrates with multiple integrated circuit devices attached.As shown in FIG. 9, these packages would have a configuration similar tothat of a single chip package, containing multiple devices 303 attachedto the substrate 301 through ball-grid arrays 302 and thermally coupledto the carbon/carbon or metal/carbon heat spreader 305 using a compliantheat-transfer medium 304. The heat spreader 305 is coupled to thesubstrate using either a sealing material 307 or through a combinationof sealing material and legs, as discussed in FIG. 8. The heat spreaderwill be further attached to a heat sink 306 to facilitate the removal ofheat from the heat spreader 305. All of the aforementioned embodimentswith regard to heat spreader construction may also apply to themulti-chip configuration.

[0046] Thus, what has been described is an apparatus for spreading heatremoved from the backside of a packaged semiconductor device. In theforegoing detailed description, the apparatus of the present inventionhas been described with reference to specific exemplary embodimentsthereof. It will, however, be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope of the present invention. The present specification andfigures are accordingly to be regarded as illustrative rather thanrestrictive.

What is claimed is:
 1. A method, comprising: placing a plurality offibers into a mold, the fibers oriented approximately in the x and ydirections; adding a second plurality of fibers; disposing a heatconductive material around the fibers; and curing the heat conductivematerial.
 2. The method of claim 1, wherein the fibers are woven.
 3. Themethod of claim 1, wherein the fibers are comprised of carbon.
 4. Themethod of claim 1, wherein the second plurality of fibers are orientedin approximately a vertical direction.
 5. The method of claim 1, whereinthe second plurality of fibers is chopped.
 6. A heat spreader,comprising: a plurality of fibers oriented approximately along ahorizontal axis; a second plurality of fibers oriented approximatelyalong the second horizontal axis, approximately perpendicular to thefirst set of fibers; a third plurality of fibers, some or all orientedapproximately in the vertical direction, approximately perpendicular tothe first and second sets of fibers; and a conductive material disposedabout the fibers.
 7. The heat spreader of claim 6, wherein the fibersare comprised of carbon.
 8. The heat spreader of claim 6, wherein thefibers are woven.
 9. The heat spreader of 6, wherein the third pluralityof fibers are chopped.
 10. A heat spreader, comprising: a first layer offibers, oriented approximately along a horizontal axis; a second layerof fibers, oriented approximately along the same horizontal axis, thesecond layer having a different fiber density than the first layer; asecond plurality of fibers in the second layer, oriented approximatelyalong a second horizontal axis, approximately perpendicular to the firstset of fibers in the second layer; a third plurality of fibers in thesecond layer, oriented approximately in the vertical direction,approximately perpendicular to the first and second sets of fibers inthe second layer; a third layer of fibers, having a fiber densitydifferent than the fiber density of the second layer; and a conductivematerial disposed about the fibers.
 11. The heat spreader of claim 10,wherein the first and third layers have a higher fiber density than thesecond layer.
 12. The heat spreader of claim 10, wherein the first andthird layers have similar fiber densities.
 13. The heat spreader ofclaim 10, wherein the fibers are comprised of carbon.
 14. The heatspreader of claim 10, wherein the fibers are woven.
 15. The heatspreader of claim 11, wherein the fibers in the first and third layersare chopped.
 16. A semiconductor package, comprising: a substrate havinga top surface; at least one semiconductor device attached to said topsurface of said substrate; a cover secured to said substrate creating aspace therebetween, said semiconductor device residing within saidspace, said cover having a flat top surface and an external bottomsurface; a first plurality of fibers disposed throughout said cover,said first plurality of fibrous structures disposed in mostly horizontaldirections in said cover; and a second plurality of fibrous structuresdisposed throughout said cover, said second plurality of fibers disposedin a mostly vertical direction in said cover.
 17. The semiconductorpackage of claim 16, wherein the cover is further comprised of acomposite material.
 18. The semiconductor package of claim 16, whereinthe fibers are further comprised of carbon.
 19. The semiconductorpackage of claim 16, further comprising a heat sink that is attached tothe flat top surface of the cover.
 20. The semiconductor package ofclaim 16, wherein the cover is secured to the substrate using a sealant.21. The semiconductor package of claim 16, further comprising aplurality of posts disposed between the substrate and the bottom plateto provide support to the cover.
 22. The semiconductor package of claim21, wherein the posts are comprised of polymeric materials.
 23. Asemiconductor package, comprising: a substrate having a top surface; atleast one semiconductor device attached to said top surface of saidsubstrate; a cover secured to the substrate creating a spacetherebetween, the semiconductor device residing within the space, thecover having a flat top surface and an external bottom surface, the topsurface and the external bottom surface being constructed of a thermalinterface material; a first plurality of fibers disposed throughout thecover, the first plurality of fibers disposed in an approximatelyhorizontal directions in the cover; and a second plurality of fibersdisposed throughout the cover, the second plurality of fibers disposedin an approximately vertical direction in the cover.
 24. Thesemiconductor package of claim 23, wherein said cover is furthercomprised of a composite material.
 25. The semiconductor package ofclaim 23, wherein the fibers are further comprised of carbon.
 26. Thesemiconductor package of claim 23, where the thermal interface materialis comprised of the same material as the composite.
 27. Thesemiconductor package of claim 26, further comprising a heat sink thatis attached to the flat top surface of the cover.
 28. The semiconductorpackage of claim 23, where the interface material is comprised of thecomposite material wherein the fiber density is greater than that of thecover fiber density.
 29. The semiconductor package of claim 24, furthercomprising a plurality of posts disposed between said substrate and saidbottom plate to provide support to said cover.
 30. The semiconductorpackage of claim 29, where said posts are comprised of polymericmaterial.